I don't know the equation but the longer the brace the shorter you have to draw the bow which reduces fatigue in more than one way. It also pushes less on the arrow which reduces the gas applied, which in turn should mean a less stiff arrow... and so on.
More forgiving is like compairing pregnat women, they're all pretty much huge. One maybe slightly smaller but it really doesn't make much of a difference. (Yes I did go down that rough road in my more niave years).
